Documentation improvements

[tatomyr]

This is a placeholder for relatively small and low-priority CLI documentation issues. Will keep it open and update as I find more.

• [ ] improve split documentation so it clarifies the intended usage (https://github.com/Redocly/redocly-cli/issues/1822)
• [x] explain how to use openapi-core features in JS - https://github.com/Redocly/redocly-cli/blob/main/packages/core/README.md
• [x] add explanation about overriding severity of configurable rules
• [ ] improve eject and other realm-related commands docs (see https://github.com/Redocly/redocly-cli/issues/1847#issuecomment-2602717316)
• [ ] clarify the filter-in decorator behaviour (https://github.com/Redocly/redocly-cli/issues/1741)
• [x] clarify the merging stragety of root and apis-level config (https://github.com/Redocly/redocly-cli/issues/1871#issuecomment-2634020349)
• [x] clarify that join doesn't run decorators (https://github.com/Redocly/redocly-cli/issues/1889#issuecomment-2642579978) since https://github.com/Redocly/redocly-cli/pull/1046 and https://github.com/Redocly/redocly-cli/pull/1507
• [x] clarify that join doesn't modify the original descriptions (see https://github.com/Redocly/redocly-cli/issues/2263#issuecomment-3190921892)
• [ ] Review our openapi-startrer docs (see this comment: https://github.com/Redocly/redocly-cli/issues/2348#issuecomment-3435126140)
• [ ] TBD

cc @JLekawa

[jeremyfiel]

• [ ] update docs for Overlay linting, decorators, preprocessors, and type extensions #1898